Plywood and other composite wood products such as LVL are produced from thin sheets of wood plies or veneers. To produce the veneer, logs are cut or peeled by sharp blades on a lathe or in other cutting machinery, producing a running ribbon or strip of veneer between one-tenth and one-quarter of an inch thick, depending on the type of plywood being made. The veneer strip is thereafter clipped to size, producing “green” or wet veneer sheets.
The green veneer sheets are dried in dryers, and glued together in layers to form a panel. Producing plywood or LVL from individual veneer sheets typically involves layering a plurality of glue-covered veneer sheets and then processing the sheets using a combination of pressure and heat to set the glue and fuse the veneer layers together.
It is important in the manufacturing process that the veneer sheets be dried to a relatively uniform moisture content, usually in the range of between 2–6% (this percentage figure being well known to those skilled in the art). The drying process stabilises the veneer dimension, strengthens the fibres, and prepares the wood for gluing. Mechanical driers are typically controlled by control of temperature, ventilation (to control humidity) and conveyer speed (to control time). In large modern driers, elaborate loading and unloading facilities are frequently provided.
It is important that the veneers not be too dry, nor too wet, or they will not bond well together. Further, if there are pockets of moisture, or areas of the veneer which are not dried to within the desired range, then “blows” can cause severe delamination in the finished plywood, which can result in product rejection.
Because the drying of the veneer sheets is such an important step in the manufacturing process, the veneer sheets are typically sorted into a plurality of subsets or “sorts” having approximately the same drying characteristics, ie. by species, thickness, heartwood or sapwood, and initial moisture content prior to drying. The driest green veneers are typically sorted together, as are the wettest, and the intermediate veneers may be sorted into one or more intermediate sorts.
The driest veneers are dried together as a batch for a certain period of time and/or at a certain temperature or humidity level to achieve the desired end moisture content. Similarly, the wettest sort is dried together (for a longer period of time, and/or at a different temperature and humidity) to achieve a relatively similar end moisture content. The intermediate sort(s) are dried for some intermediate time, and/or at another different temperature and/or humidity.
Therefore, veneer drying is a very important aspect of production of wood composite products from veneers, and moreover, veneer sorting is a very important factor relating to drying. One continuing problem in sorting green veneer, however, relates to the determination of its initial moisture content in determining which “sort” a particular sheet should be directed to.
It has been the case in the past that the relative sort point of moisture content of a particular sheet of green veneer will simply be estimated by a human operator of a sorting system. After years of experience, an operator may have the skill to roughly sort sheets into appropriate subsets, knowing the species of the wood being dried and its thickness, by knowing whether the sheet comes from sapwood or heartwood, and by looking at the sheet as it passes downstream from the cutting apparatus. Unfortunately, the human operator has little time to see the sheet in a typical fast-moving system, and it is not uncommon for sheets to be misdirected into inappropriate sorts, even by the most experienced operator.
The problem with inappropriately directing sheets into the wrong sort is that if the sheet is dried for a longer period of time than necessary to reach the desired moisture content, bonding sites on the surface of the veneer can be destroyed or deactivated, thus creating bonding problems between plies. If the sheet is dried for a shorter period of time than necessary, it will still be too wet to use.
In a typical mill which relies on an operator's expertise in judging the initial moisture content of veneer sheets, it is more typical (and indeed prudent to avoid wastage from over drying) that the operator will direct a sheet to a “shorter time” sort than would be more appropriate (ie. a wet sheet will be placed inappropriately in a sort which is not dried for very long). This avoids over drying, but it does not allow the sheet to dry to the desired moisture in one pass through the dryer. All sheets are checked for moisture content after they are dried, and the wet ones are returned to the dryer as “re-dries”. An operator will adjust the variables in the drying system during the drying process to try to reduce the amount of “re-dry” iteratively: when too many sheets come out of the dryer still wet, the operator adjusts the time, temperature and humidity to reduce the percentage of “re-dries” for the remainder of the sheets in the batch.
However, even with this iterative intervention by a human operator, the percentage of “re-dry” in a typical veneer drying system is quite high, and this leads to increased costs of production in increased machine time, increased operator time, and increased energy usage.
It is important, therefore, to try to reduce the amount of re-dry in a veneer drying system. To reduce the amount of re-dry, it would be desirable to place certain sheets which would otherwise end up out of the drier as re-dry into a sort which has wetter sheets in it (to subject the sheet to a longer drying time, or higher temperature or lower humidity). For this, it would be useful to have a good understanding of the nature of the initial moisture content of the veneer sheet.
Attempts have been made in the past to better measure the initial moisture content of green veneer sheets, in an effort to minimize the effect of the necessity for human intervention in the sorting process. For example, certain attempts have been made to determine the moisture content of a veneer sheet by measuring the electrical or dielectrical properties of the wood. It has been found, however, that the sensors used for this purpose are accurate only when the moisture content is low (ie. below 30%).
Most commonly today, there are also Radio Frequency-based (“RF”) sensor systems in use. These sensors use radio frequencies to measure moisture content of sheets of veneer. They too, however, are not accurate at higher moisture ranges (perhaps >70%), but more importantly, RF sensors are highly sensitive to operating conditions. The accuracy of RF sensor measurements are significantly affected by veneer shape and flatness, and by wood grain angle, and by the distance between the RF source and the face of the veneer sheet. Small distance changes (in the order of a millimetre or two, or even less) between the RF source and sensor head and the sheet face cause large variations in readings. In most drying and sorting systems, veneer sheets are moved along conveyors rapidly and “bounce” along the conveyor such that there are widely varying distances between the sheet and the RF sensor head as the sheet passes the sensor head, leading to erroneous readings.
There remains a need, therefore, for a better system for sorting green veneer more appropriately and more accurately into sorts or subsets for drying in a dryer, thereby reducing the re-dry amount and increasing productivity, reducing energy consumption, and providing better quality dried veneer for use in plywood. The inventors believe that the key to this desired system is improved moisture measurement accuracy.